High-bandwidth optical communications relay architecture

ABSTRACT

A free space optical communication system ( 100 ) and method including a constellation of several satellites ( 102 ). Each of satellites including: several inter-satellite optical telescopes ( 204 ) for optical communication with multiple neighboring satellites, each inter-satellite optical telescope is capable of adjusting its elevation angle; and several up/down link optical telescopes ( 206 ) for optical communication with multiple ground sites. As the constellation passes a given ground site, some of the up/down-link telescopes of a given satellite are configured to track at least two respective ground optical telescopes of the given ground site and send data to the ground optical telescope with the clearest line of sight to the given satellite. Moreover, each of the satellites includes optical circuitry ( 208, 210, 212, 216 ) for optically processing and switching incoming and outgoing optical signals without converting the optical signals into electrical signals.

FIELD OF THE INVENTION

The present invention relates to a high-bandwidth optical communicationsrelay architecture.

BACKGROUND

The use of the Internet worldwide is ever increasing with a high growthrate in the developing countries around the world. However, manyemerging business centers in regions near the Equator are handicapped bypoor connectivity to the Internet. These centers are typically locatedin countries with limited national high bandwidth networkinfrastructure, and sometimes surrounded by either hostile neighbors orinhospitable terrain that makes terrestrial and undersea cableconnections impractical.

Nevertheless, there is a continuing demand for high bandwidthconnectivity to the Internet in these countries. Many of the mostrapidly growing markets are both near the Equator and poorly connectedvia undersea cables. For some of the larger countries, the internalnetwork infrastructure is relatively primitive. Furthermore, naturaldisasters can also disrupt connections, and the ability to rapidlyreconfigure a communication network to reconnect the affected areas canbe extremely valuable. In addition to the underserved markets, the majorglobal telecom carriers of significant and growing wholesale bandwidthhave needs for backup and replacement bandwidth to maintain Quality ofService agreements.

Geostationary Earth Orbit (GEO) communication satellites have inherentlyhigh latency, while other satellite communication networks suffer fromsome combination of limited worldwide connectivity, low bandwidth, orcost. The GEO satellites offer coverage of a reasonably large fractionof the Earth per satellite but have long communication paths (˜36,000km) resulting in a signal latency of at least 120 msec per path.Moreover, multiple bounces may be required to provide routing, andconnection between ground sites not within footprint of same satellitemay require ground connections. Additionally, GEO communicationsatellites are currently restricted to Radio Frequency (RF) signals,which limit available bandwidth to a range of hundreds of MHz to a fewGHz. Furthermore, multiple beams need to be used to provide relativelyhigh total throughput per satellite (72 beams at 48 Mbps is typical, for3.4 Gbps per satellite).

The “Other 3 Billion” (O3B) program is attempting to serve the samegeneral equatorial region using Radio Frequency (RF) signals. As aresult of RF usage, O3B has severe bandwidth restrictions. O3B uses aMedium Earth Orbit (MEO) constellation of 8 to 12 satellites inEquatorial orbit at 8,000 km. Each satellite will have up to 10 RF linksthat will (eventually) be capable of up to 1.2 Gbps per channel. Theconstellation is rated at 70 total ground sites at 1.2 Gbps per groundsite, or 84 Gbps total, and the satellite network is divided into 7regions, with a single gateway per region. O3B also has nointer-satellite links, so communicating across regional boundariesrequires multiple bounces.

The Iridium™ constellation simply doesn't have the bandwidth to addressthe same market. Iridium's™ Low Earth Orbit (LEO) constellation has analtitude of about 780 km, which limits access per satellite.Accordingly, a constellation of 66 active satellites is used to provide24/7 coverage of the entire world. Use of L-band in LEO constellationlimits the bandwidth of satellite phones to less than 1 Mbps. Gatewaylinks offer 10 Mbps of bandwidth to a few selected locations. Moreover,inter-satellite links are RF, with substantially limited bandwidth.

Some limited experiments were conducted for free-space opticalcommunication (FSO), also sometimes referred to as laser communication,or lasercom for short, by the National Aeronautics and SpaceAdministration (NASA) around 2005, in the NASA Mars TelecommunicationOrbiter program. However, these experiments proved to have limitedcoverage duration, limited connectivity, and usually limited bandwidthof about 5-10 Gbps of upper limit per link. No commercial viability wasthe conclusion of the program.

Several attempts have been made to establish a space-based lasercommunication network. One such network was the TransformationalCommunication Architecture (TCA), which was designed around a backboneof GEO satellites with inter-satellite links, and laser links to otherspacecrafts and to airborne platforms and ground sites. The estimatedcost of TCA was so high that it could not survive and was cancelled atits onset.

All prior attempts at lasercom in space have used an optical toelectrical to optical (O-E-O) approach, with the incoming optical signalconverted to an electrical signal and then converted back to an outgoingoptical signal. The approach has the advantage that the signal canundergo a full re-amplification, re-shaping and re-timing (3R)regeneration on-board while it is in the electronic domain, but thesize, weight, and especially power of the hardware has been a severechallenge. Much of the work has also concentrated on using satellites inGEO, for which the range is as much as 6 times further than the MEOsatellites.

SUMMARY

In some embodiments, the present invention is a free space opticalcommunication system which includes a constellation of severalsatellites. Each of satellites includes: several inter-satellite opticaltelescopes for optical communication with multiple neighboringsatellites, and each inter-satellite optical telescope is capable ofadjusting its elevation angle to accommodate changes in the number ofsatellites in the constellation. Each of satellites further includes:several up/down link optical telescopes for optical communication withmultiple ground sites, where each ground site has two or more groundoptical telescopes. As the satellite constellation passes a given groundsite, one or more of the up/down-link telescopes of a given satelliteare configured to track at least two respective ground opticaltelescopes of the given ground site and send data to the ground opticaltelescope with the clearest line of sight to the given satellite.Moreover, each of the satellites includes optical circuitry foroptically processing and switching incoming and outgoing optical signalswithout converting the optical signals into electrical signals.

In some embodiments, the present invention is a method for free spaceoptical communication in a constellation of a plurality of satellites.The method include: using a plurality of inter-satellite opticaltelescopes in each of the plurality of satellites to opticallycommunicate with multiple neighboring satellites; using a plurality ofup/down link optical telescopes in each of the plurality of satellitesto optically communicate with multiple ground sites, each ground sitehaving two or more ground optical telescopes; and tracking, by a givensatellite, at least two respective ground optical telescopes of a givenground site and sending data to a ground optical telescope with theclearest line of sight to the given satellite, wherein each of theplurality of satellites optically processes and switches incoming andoutgoing optical signals, without converting the optical signals intoelectrical signals.

In some embodiments, the present invention is a free space opticalcommunication system which includes a constellation of severalsatellites. Each of satellites includes: several inter-satellite opticaltelescopes for optical communication with multiple neighboringsatellites, and several up/down link optical telescopes for opticalcommunication with multiple ground sites. Each of the plurality ofsatellites includes optical circuitry for optically processing andswitching incoming and outgoing optical signals without converting theoptical signals into electrical signals.

In some embodiments, each inter-satellite optical telescope may utilizecircular polarization or spectral diversity to provide dual opticalsignal paths. Furthermore, the one or more of the up/down-linktelescopes of a given satellite may be configured to continuously and inreal time track at least two respective ground optical telescopes of thegiven ground site, for example, by using an optical beacon. In someembodiments, a single up/down-link telescope of the given satellite maybe configured to track said at least two respective ground opticaltelescopes of the given ground site using circular polarization orspectral diversity.

In some embodiments, each inter-satellite optical telescope may includebeam steering mirrors to compensate for jitter and orbit differences ofsaid each inter-satellite optical telescope. In some embodiments, eachup/down link optical telescope may include dual internal steeringmirrors to maintain track on said at least two respective ground opticaltelescopes of the given ground site.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention, and many of theattendant features and aspects thereof, will become more readilyapparent as the invention becomes better understood by reference to thefollowing detailed description when considered in conjunction with theaccompanying drawings in which like reference symbols indicate likecomponents, wherein:

FIG. 1 shows an exemplary MEO constellation of a plurality of satelliteswith optical communication, according to some embodiments of the presentinvention.

FIG. 2 is an exemplary layout view of a satellite payload includingcross link and up/down telescopes, according to some embodiments of thepresent invention.

FIG. 3 is an exemplary block diagram of an optical communication paththrough a satellite payload, according to some embodiments of thepresent invention.

FIG. 4 is a simplified block diagram for an on-board optical hardwaresystem, according to some embodiments of the present invention.

FIG. 5 is an exemplary block diagram for an on-board optical hardwaresystem, according to some embodiments of the present invention.

FIG. 6 is an exemplary block diagram for an on-board optical hardwaresystem, according to some embodiments of the present invention.

DETAILED DESCRIPTION

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments thereof areshown. The invention may, however, be embodied in many different formsand should not be construed as being limited to the embodiments setforth herein. Rather, these embodiments are provided so that thisdisclosure is thorough and complete, and will fully convey the conceptof the present invention to those skilled in the art.

In some embodiments, the present invention is directed to ahigh-bandwidth transparent optical communication relay architecture thatprovides a novel approach for offering many of the world's most rapidlygrowing business centers a dramatically improved throughput ofcommunication signals, including improved connectivity to the Internet.The space optical hardware is designed to be as agnostic as possibleabout future evolution of optical communication standards, so it doesn'tbecome obsolete over time. The ground hardware can be upgradedincrementally to support higher bandwidths or any changes in thestandards. The network is highly flexible, with multiple redundant pathsand rapid reconfiguration.

In some embodiments, the present invention is directed to a novelapproach (e.g., a constellation of MEO satellites) to satisfy the needfor an optical relay on a space platform that is capable of linking aredundantly connected ring of MEO satellites to a network of groundsites. This constellation of MEO satellites can do so transparently andindependent of the optical format and modulation scheme, and preferablythe signal is within the C-band or L-band spectral bands, with possibleexpansion to other optical communication bands. Using communicationstandards, some embodiments are capable of providing a total throughput(counting both directions of each link) of at least 800 Gbps to groundsites, with the ability to pass at least 1,600 Gbps of data to and fromneighboring satellites. The payload is configured to support growth ofthe constellation, with new satellites launched to easily add capacity,and the payload can rapidly reconfigure the network to drop any failedsatellite out of the network.

FIG. 1 shows an exemplary MEO constellation 100 of a plurality ofsatellites 102, according to some embodiments of the present invention.As shown, eight satellites 102 (8-ball constellation) are arranged andnetworked together to provide a continuous coverage of a band of theearth, especially around the equatorial orbits. Although, eightsatellites are shown as an example, the present invention is not limitedto eight satellites and a different number of satellites, for example,four, sixteen or other number of satellites can be used for morecoverage time per satellite and/or redundancy purposes. Each satelliteof the MEO constellation is optically coupled to multiple nearestneighbors (for example, 4 or more, except in the minimal 4-ballconstellation, where only 2 neighbors are visible) using inter-satellitelasercom (ISL) optical telescopes. In some embodiments, circularpolarization or spectral diversity is used to provide dual opticalsignal paths per telescope. In some embodiments, circular polarizationis used to separate transmitted signals from received signals.

Different spectral regions can also be used to allow four or more pathsper ISL optical telescope, with minimal impact on the complexity of thenetwork channel assignment. The ISL optical telescopes are capable ofadjusting their elevation angle to allow (accommodate) a different angleof optical communication to accommodate adding or removing satellites(i.e., changes in the number of the satellites in the constellation)from the ring (constellation) and re-phasing the satellites that arecurrently in use.

For example, in the case of a new satellite being launched into theconstellation, one or more commands for modifying the trajectory andreconfiguring the inter-satellite and ground optical communications(telescopes) are sent, for example, from a ground telescope in a groundsite to each satellite.

In some embodiments, each satellite is connected to multiple groundsites using up/down-link optical telescopes. The minimum possibleconfiguration is a single up/down-link telescope per satellite, however,multiple telescopes increase the overall capacity of the network andwould provide a larger revenue stream. The host satellite can easilysupport at least four up/down-link telescopes, although six or eighttelescopes may be preferable in some embodiments. Connections arescheduled so that at least one up/down-link telescope is free wheneverthe ground connection from a preceding satellite in the constellation isnearing an end, allowing the network to establish a new connectionbefore breaking the old one, that is, a “Make-Before-Break” scheme.

Site diversity on the ground is used to mitigate weather outages, withmultiple (two or more) terminals (ground optical telescopes) inrelatively close proximity to each other, for example, within a fewhundred kilometers of the associated ground gateway. Two of these groundterminals/telescopes are selected for each pass of an opticallyconnected satellite to the ground site, based on predicted cloud-freeline of sight probability for the pass. These two ground terminals maybe tracked by separate up/down-link telescopes on the satellite, but itwould also be possible to utilize a dual-tracking system with a field ofview large enough to cover both ground terminals simultaneously by asingle telescope. In some embodiments, dual polarization is used as oneapproach to distinguishing between the signals from the two groundterminals when spatial separation is inadequate. In some embodiments,different spectral bands can be used for the beacons.

In addition, each ground site would have at least two up/down-linktelescopes so that new connections can be established before the old oneis broken, as the satellite constellation passes the ground site. Thisway, the satellite constellation has a high availability by using sitediversity, with monitoring and real-time switching between separateground terminals supporting a single gateway/site. During a pass, theup/down-link telescopes continuously and in real time track both currentground sites/terminals, using an optical beacon, and send data to theone with the clearest line of sight. The multiple (e.g., two or more)ground terminals in relatively close proximity to each other are indirect communication with a shared gateway via wired or wireless,electrical or optical communication schemes. The gateway may be locatedin a carrier hotel or other site with multiple connections to local highspeed Internet networks.

In some embodiments, each of the ground terminals includes a telescopeand antenna system for steering the optical beams at the one or more ofthe satellites. In some embodiments, ground-based gimbaled lasercomterminals/telescopes track individual satellites during each pass. Insome embodiments, a network operations center sends up one or moreswitching commands to configure the data paths to maintain continuousconnectivity between desired ground sites, with extra links used to makenew connections before the old ones are dropped as the satellites orbitaround the earth.

Although the satellite constellation of FIG. 1 is described with respectto a MEO orbit, a combination of GEO and MEO orbit satellites are alsopossible and are within the scope of the present invention. The additionof one or more GEO satellites to the architecture may be done inmultiple ways. In a simple, but limited approach, the gateways for theMEO relay constellation would be co-located with terminals for the GEOsatellites. This approach uses the MEO constellation to pass data fromone GEO satellite to another. A more flexible approach can add one ormore GEO-link telescopes to each MEO satellite, with a field of regardthat is large enough that at least one MEO satellite will always be ableto communicate with any satellite in GEO orbit. Given the increasedrange, the amplifiers designed to close the MEO links would be capableof supporting 3-5 Gbps going to and from GEO. Higher data rates can beachieved with increased power output from the amplifiers. Because thehardware operates in a transparent mode, there is no need to make anysignificant changes to support this different data rate and range. Insome embodiments, it is also be possible to add or switch to an RFcommunication mode for uplink and downlink to GEO satellites, using aconformal electronically steered array (ESA). In some embodiments,because the RF signals are only be used to communicate from onesatellite to the other, the frequency allocation issues are simplifiedand the utilized frequency can be selected as one that does notpenetrate the atmosphere.

In some embodiments, a combination of Low Earth Orbit (LEO) and MEOorbit satellites are also possible and are also within the scope of thepresent invention. There are also multiple ways to add one or more LEOsatellites to the constellation of the present invention. In every case,the focus is on one-directional data flow, from LEO satellitescollecting data at relatively high data rates to a gateway connected tothe ground processing center for the satellites. A simple approach is toadd a compatible lasercom transmitter to the LEO satellite. Theup/down-link beam directors onboard the relay satellites are capable oftracking the LEO satellite, when it is within about 20 degrees of theequator. For sun-synchronous or nearly polar satellites, this wouldprovide two communication windows per orbit, each representing roughly10% of the orbital period of the satellite. The connection time wouldincrease for lower inclination LEO orbits. A store-and-dump strategyusing an on-board data storage unit may be used.

A somewhat more complicated approach would add a ring of LEO satellitesin the same orbit plane to the constellation of the present invention.Each LEO satellite has a pair of inter-satellite links communicatingwith its nearest neighbors, and an uplink lasercom transmitter to theMEO ring. The LEO satellites would receive data from the satellitepreceding it in orbit, add its own data, and then transmit the result tothe following satellite. When the data stream reaches one of the twosatellites in the LEO ring that is connected to the MEO ring, the datais then sent up to the MEO satellite and relayed to a desired gatewaywithin the normal MEO coverage zone. This approach results in all LEOsatellite data reaching the ground with a latency of less than a second.It would also reduce or eliminate the need for onboard data storageunits for the LEO satellites. A mixed approach is also possible, inwhich a partial LEO ring is established to increase the availableconnect time. As long as any of the satellites in the partial ring isable to contact a MEO relay satellite, the data from all of thesatellites reach the gateway in a fraction of a second.

FIG. 2 is an exemplary layout view of a satellite payload 200 includingcross link and up/down telescopes, according to some embodiments of thepresent invention. As shown, a plurality of inter-satellite (cross link)telescopes 204 a to 204 c (four shown in this exemplary figure) areinstalled on an exterior portion of each satellite for inter-satelliteoptical communications. In this example, telescopes 204 a and 204 b arelocated at the right side of the satellite and communicate with one ormore of its nearest neighbor satellites on its right side. For example,204 a would point to and communicate with the nearest neighboringsatellite (i.e., adjacent satellite) on that side and 204 b would pointto and communicate with the next nearest neighboring satellite (i.e.,two satellites away) on that side. Similarly, telescopes 204 c and 204 dare located at the left side and communicate with one or more of itsnearest neighbor satellites on its left side. Each of the cross linktelescopes are capable of being selectively steered for calibrating theoptical communication with the neighboring satellites, and/or in thecase of a new satellite being added to the constellation, forestablishing new optical communication with the new satellite. That is,the inter-satellite optical links connect the respective satellite in aredundant network.

In some embodiments, the cross link telescopes 204 a to 204 d includeadjustable elevation settings to track the neighboring satellites in theconstellation. In some embodiments, two nearest neighbors and twonext-nearest neighbors are utilized for inter-satellite communications.Beam steering mirrors used to compensate for host satellite jitter andslight orbit differences. Elevation adjustment is used on an infrequentbasis to add or drop satellites into the constellation and communicationring. Since the cross link telescopes are a shared resource, severalmethods are appropriate for using polarization and coarse wavelengthseparation to combine signals into a cross-link and then separate thedata after transmission. In some embodiments, four or more signalbundles share the same cross-link telescope.

Additionally, there are a plurality of up/down link telescopes 206 a to206 g (six shown in this exemplary figure) installed on the exteriorportion of each satellite for ground communication and site diversity.In some embodiments, each up/down link telescope supports at least asingle high-bandwidth (e.g., 100 Gbps) bi-directional connection betweenground sites around the world. Another up/down link telescope either onthe same satellite or a connected satellite is used for the other end ofthe connection. With eight satellites and six up/down-link telescopesper satellite, the network can support up to 24 of the high-bandwidthbi-direction connections.

In some embodiments, the up/down link telescopes 206 a to 206 g aregimbaled telescopes or telescopes with coelostats on each satellite totrack a ground site and establish a high-bandwidth link. In someembodiments, Dense Wavelength Diversity Multiplexing (DWDM) is used toprovide bi-directional 100 Gbps (or more) in bandwidth links with eachground site. Polarization and/or wavelength diversity is also used toisolate the two data streams. The site diversity is used to reduceoutages due to clouds within the line of sight. Each telescope has dualinternal steering mirrors to maintain track on two receive groundtelescopes within a 100 km radius of a central point, which is trackedby the gimbal or coelostat. These two ground telescopes can be selectedfrom a larger set before each satellite pass.

In some embodiments, the up/down link telescopes are small opticaltelescopes (for example, about 10 cm diameter aperture) either on gimbalor using a coelostat to track the ground sites. Multiple beam steeringmirrors and control loops allow each up/down link telescope tosimultaneously track two terminals within a 100 km radius of the groundsite, which may be selected on each pass from a larger list of availableterminals. In some embodiments, the ground optical telescopes arelarger, for example, nominally 40 cm diameter, which may eliminatecoelostats as an option for steering the beams from these largetelescopes. In some embodiments, the inter-satellite links areestablished by larger, for example, about 30 cm, telescopes that use afast beam steering mirror to compensate for platform jitter and slightvariations in orbit, with an elevation mechanism used to re-point alongthe orbit plane any time new satellites are added to the ring or failedsatellites are removed from it. The cross link telescopes used fornearest neighbor connections may be smaller than those used for moredistant next-nearest neighbors, to keep the rest of the hardwareidentical and reduce payload mass.

The optical on-board hardware (payload) of each satellite includes aplurality of optical pre-amplifiers 212, an optical switch matrix 208,one or more main amplifiers 210, a plurality of power amplifiers 216, aCommand, Control, And Telemetry (CC&T) subsystem 214, and a power supply218. In some embodiments, the optical pre-amplifiers 212 and poweramplifiers 216 support a fixed number (for example, 10) of independent10 Gbps channels with acceptable cross-talk and sufficient totalamplification to provide acceptable signal-to-noise ratio (SNR) orphotons per bit, at each receiver. In other embodiments, the laser pumppower is scalable so that the number of channels per amplifier can beadjusted to accommodate different demands for bandwidth. The amplifiersmay be Erbium-Doped Fiber Amplifiers (EDFAs), Planar Waveguides (PWGs),Raman amplifiers, Semi-Guiding High Aspect Ratio Core (SHARC) fiberlaser amplifier, other technologies, or a combination thereof.

In some embodiments, commercial standards are used to the extentpossible, with space qualification of commercial, off-the-shelf (COTS)parts, the desired approach to space hardware, and direct use of COTSparts on the ground. In some embodiments, C-band or C and L-band opticalamplifiers use Planar Waveguide (PWG) or related technology. In someembodiments, International Telecommunication Union (ITU) standard 50 GHzchannel separation for the payloads is used, with possible simpleupgrades in future as the standards evolves and commercial hardware isdeveloped to support it. In some embodiments, a power interface isconfigured to connect to a (standard) power bus of a satellite so thatthe optical hardware of the present invention can fit into a “standard”or pre-existing satellite platform with certain power limitations. Ingeneral, the satellite optical hardware is as transparent and agnosticas possible to specific implementation details, so that all suchupgrades can be achieved on the ground.

Each of the plurality of satellites includes optical circuitry/hardwarefor optically processing and switching incoming and outgoing opticalsignals without converting the optical signals into electrical signals.The on-board hardware receives an incoming optical data stream from theground and/or one or more neighboring satellites, optically regeneratesit, uses optical switches to direct it to the desired (selected) outputpath, and sends it toward its final destination (ground and/or one ormore neighboring satellites). Regeneration of the incoming optical datastream includes re-amplification (by the optical pre-amplifiers 212, themain amplifiers 210 and the power amplifiers 216), all in opticaldomain. That is, the amplification of the optical data stream isaccomplished without ever converting to electrical signals within eachsatellite payload, and transparently to data modulation schemes. Theon-board hardware is capable of operating in C-band, L-band and otheroptical bands, and reshaping and re-phasing the optical data stream.

In some embodiments, channel separation of about 50 GHz with DenseWavelength Division Multiplexing (DWDM) is used to provide at least 8010-GHz channels. However, more channels and higher bandwidth per channel(using more complicated modulation schemes) are possible and are withinthe scope of the present invention. The optical switch matrix 208 allowseach optical input to be optically coupled to any other output channel.In some embodiments, the optical switch matrix 208 is capable ofswitching whatever signal it receives on each input, including entirebundles of channels. In some embodiments, de-multiplexing, switching atthe individual channel level, and re-multiplexing are performed to allowswitching each individual channel. The on-board optical switch matrix208 also allows establishing and updating network optical paths as thesatellite constellation passes over the ground sites.

One or more main lasers 210 are used on each satellite as part of theamplification chain for the optical signals on each satellite. In someembodiments, where the channels are all multiplexed together, the mainamplifier may require as much power as the final power amplifiers. Inother embodiments, in which each connection has its own amplifier chain,the main amplifiers require significantly less power than the finalpower amps.

FIG. 3 is an exemplary block diagram of an optical communication channel300, part of which travels through a payload of the satellite, accordingto some embodiments of the present invention. As shown, an opticalsignal is generated by a ground user in block 302. The generated opticalsignal is pre-amplified in block 304 and power amplified in block 306,before it is provided to a ground terminal optical transceiver 308. Theground terminal optical transceiver 308 uses one of the ground terminaltelescopes to transmit the amplified optical signal to the selectedsatellite. The transmitted optical signal is received by the satellitetransceiver telescope 310 and amplified by the satellite preamp 312 andmain amp 314. The amplified optical signal is then directed to a desiredlocation, for example, the ground terminal transceiver 328 or theneighboring satellite transceiver telescope 318, by the on-board opticalswitch 315. After the path has been selected, the optical signal isamplified one more time by the optical power amplifier 316 associatedwith the selected telescope.

The optical switch network may be in various forms, including a simpleN×N cross-connect optical switch that is transparent to the opticalsignal content or a de-multiplexer followed by an 80×80 non-blockingcross-connect optical switch that allows distribution of the data streamfrom each origin to multiple destinations (targets). In someembodiments, the optical switch network may be placed immediately afterthe preamplifiers, to reduce the power handled by the switch. In someembodiments, the main amplifier and power amplifier may be combined intoa single higher gain power amplifier.

When the optical signal is received by the neighboring satellitetransceiver telescope 318, it is amplified (in the neighboring satellitetransceiver) by optical preamplifier 320 and optical main amplifier 322.An on-board optical switch 324 (of the neighboring satellite) redirectsthe signal to the ground terminal transceiver 328, after it is amplifiedby the on-board optical power amplifier 326. The received optical signalis then amplified by an optical pre amplifier 330 and an optical poweramplifier 332, before it is sent to the user 334 for further processing.In some embodiments, the ground transceivers 308 and 328 are at twodifferent sites. The satellite transceiver telescope 310, theneighboring satellite transceiver telescope 318, and the groundtransceivers (telescopes) 308 and 328 are capable of pointing to andtracking their target telescopes, as indicated by “APT” (Acquisition,Point, Track) designation, in FIG. 3. This optical communication channelarchitecture is also sometimes referred to as a bent pipe. As statedabove, the optical signal is not converted to electrical signals, so itis as if the optical signals are simply traveling through a bent pipethat receives the signal and changes its direction to another groundstation or satellite.

FIG. 4 is a simplified block diagram for an on-board optical hardwaresystem 400, according to some embodiments of the present invention. Thisfigure is simplified to illustrate only one destination from the opticalswitch 408. As shown, an incoming optical signal is received by anoptical beam expander 414 and pre-amplified by an optical preamplifier412. In some embodiments, each incoming bit in the incoming signalcontains ˜400 photons, based on the link budget. To achieve that samelevel at the next stage, either on the ground or on another satellite,the signal needs to be amplified by a factor of roughly 4 million. Thepre-amplified optical signals may then be combined by a multiplexer. Thecombined signals for the individual channels are then amplified by anoptical main amplifier 404 and de-multiplexed by De-Mux 406 and then fedto an optical (crossbar) switch 408 to be directed to a selecteddestination by the beam expander 414, after it is amplified by a finalpower amplifier 410. In some embodiments, the multiplexer and de-Mux canbe eliminated because separate main amplifiers are used for eachpreamplifier.

The optical amplifiers are designed to be extremely low noise, so thatphoton shot noise is the dominant noise source. The optical amplifiersmay include one or more optical filters to reduce any noise that may beincluded in the optical signal from a previous stage. Threeamplification stages are regarded as optimal, with the preamp optimizedfor low noise, the final power amp optimized for wall-plug powerefficiency (electrical to optical conversion efficiency), and the mainamp balancing the two requirements. In some embodiments, the opticalbeam expanders are configured to use Dense Wavelength DiversityMultiplexing (DWDM) to provide bi-directional 100 Gbps (or more) inbandwidth links with each ground site.

In some embodiments, Planar Waveguide (PWG) lasers are used for allthree types of the optical amplifiers, however, other options, forexample, Erbium Doped Fiber Amplifier (EDFA) and Semi-Guiding HighAspect Ratio Core (SHARC) fiber laser amplifiers can also be used. Anexemplary SHARC laser amplifier is disclosed in a co-owned U.S. PatentApplication No. 2009/0041061, filed on Aug. 9, 2007, the entire contentsof which is hereby expressly incorporated by reference. In someembodiments, the beam expander 414 is a 10-cm up/down beam expander withgimbal and fast steering mirror (FSM). In some embodiments, the opticalpreamplifier 412 is capable of amplifying the incoming signal by afactor of bout 4000× (gain) and output 1 mW per channel signals from0.25 μW per channel input signals. In some embodiments, the optical mainamplifier 404 has a gain of about 250× and is capable of taking 80channels of about 0.4 mW per channel as input and output 80 channels at100 mW per channel, or 8 W total. In other embodiments, there aremultiple main amplifiers, each paired with a preamplifier and input tothe N×N optical cross-bar switch, and capable of taking 10 inputchannels of about 0.4 mW per channel and outputting 10 channels of 100mW per channel, or 1 W total. In some embodiments, the optical poweramplifier 410 has a gain of about 25× and is capable of taking 10channels of about 0.4 mW per channel as input and output 10 channels at1 W per channel.

The optical (crossbar) switch is capable of performing differentswitching approaches, which allows a fully transparent point-to-pointconnections or a more flexible mesh connection between all of the groundsites. Each input can be connected to any output, without blocking theother inputs. In some embodiments, the optical switch usesMicroelectromechanical systems (MEMS) technology, with multiple smallmirrors tilting as commanded to reflect each optical signal from itsinput to the desired output. In some embodiments, a low-losspiezoelectric switch is used.

FIG. 5 is an exemplary block diagram for an on-board optical hardwaresystem 500, according to some embodiments of the present invention.Here, a beam expander 502 is used for inter-satellite communicationswith a nearest neighboring satellite and a beam expander 504 is used forinter-satellite communications with next nearest neighboring satellite,down stream. Similarly, a beam expander 506 is used for inter-satellitecommunications with a nearest neighboring satellite and a beam expander508 is used for inter-satellite communications with next nearestneighboring satellite, up stream. In some embodiments, the beamexpanders 502, 504, 506 and 508 are each a 30-cm ISL beam expander withmulti-positional mounting with FSM. In some embodiments, the ISL beamexpanders used for the nearest neighbor connections (502 and 506) arehalf that size, that is, 15 cm. Additionally, four beam expanders 510are used for ground communications with two or more optional additionalbeam expanders 512 for redundancy purposes. As shown, each beam expanderis associated with a power amplifier and a pre-amplifier.

The functions of the Mux, Main Amplifier, De-Mux and the opticalcrossbar switch 514 are similar to those described with respect to FIG.4. Each of the amplifiers may include one or more optical filters toreduce signal noise. In some embodiments, the on-board optical hardwareincludes payload structure and thermal sub-systems 516, payloadelectronics 518 and payload software 520. These are payload functions,with the payload structure providing structural support and the thermalsub-system providing temperature control. The payload electronicsaccepts network switching commands and controls the configuration of theoptical crossbar switch, and the payload software interprets commandstrings and translates them into the proper switch instructions. Asshown, the optical amplification is distributed with a plurality of highgain pre-amplifiers, one or more main amplifiers, and a plurality ofpower amplifiers. In some embodiments, some reshaping of the pulses maybe added to the payload hardware, but Error Detection and Correction(EDAC) and Doppler correction are deferred until the signal reaches itsground destination.

FIG. 6 is an exemplary block diagram for an on-board optical hardwaresystem 600, according to some embodiments of the present invention. Inthese more simplified embodiments, there is no multiplexer orde-multiplexer to separate individual channels from channel bundles.Accordingly, processing (amplification and switching) is performed oneach channel bundle, rather than each individual channel. The beamexpanders, preamplifiers and power amplifiers are similar to thosedepicted in and described with respect to FIG. 5. In these embodiments,there are separate main amplifiers for each connection being relayedthrough the satellite, so the main amplifiers can be designed with lowerpower requirements and a reduced heat load per amplifier. The opticalcrossbar switch may be similar to the N×N version shown in FIG. 5.

The implementation of an all-optical relay in space reduces the size,weight, and power of the payload. The optical switches in space, withvarying levels of switching complexity, allow fully transparent or fullyflexible worldwide network connectivity. The use of multipleinter-satellite link telescopes adds network redundancy, while theaddition of an elevation adjust mechanism to these telescopes allows newsatellites to be added at any time and failed satellites to be removedfrom the network. The use of multiple up/down link telescopes allowseach satellite to support multiple ground sites within its moving areaof responsibility, while the use of dual line-of-sight control loopswithin each telescope's field of view adds local area site diversity toreduce the impact of clouds. That is, each up/down link telescope cansimultaneously track two local area sites, and use whichever one has theclearer line of sight.

Furthermore, the on-board optical hardware of the present inventionprovides transparent relay of the incoming bit stream and accommodatesevolution and revisions in standards over the operational life of thehost satellites, because the optical bent pipe is independent of anystandards and therefore any changes in the standard is accommodated bythe changes in the ground hardware.

It will be recognized by those skilled in the art that variousmodifications may be made to the illustrated and other embodiments ofthe invention described above, without departing from the broadinventive step thereof. It will be understood therefore that theinvention is not limited to the particular embodiments or arrangementsdisclosed, but is rather intended to cover any changes, adaptations ormodifications which are within the scope and spirit of the invention asdefined by the appended claims.

What is claimed is:
 1. A free space optical communication systemcomprising: a constellation of a plurality of satellites, each of theplurality of satellites comprising: a plurality of inter-satelliteoptical telescopes for optical communication with multiple neighboringsatellites, and wherein each inter-satellite optical telescope iscapable of adjusting its elevation angle to accommodate changes in thenumber of satellites in the constellation, and a plurality of up/downlink optical telescopes for optical communication with multiple groundsites, each ground site having two or more ground optical telescopes,wherein as the satellite constellation passes a given ground site, oneor more of the up/down-link telescopes of a given satellite areconfigured to track at least two ground optical telescopes of the givenground site, determine a line of site for each of the at least twoground optical telescopes of the given ground site, and send data to aground optical telescope with the clearest line of sight to the givensatellite.
 2. The system of claim 1, wherein each inter-satelliteoptical telescope utilizes circular polarization or spectral diversityto provide dual optical signal paths.
 3. The system of claim 1, whereinsaid one or more of the up/down-link telescopes of a given satellite areconfigured to continuously and in real time track at least tworespective ground optical telescopes of the given ground site.
 4. Thesystem of claim 1, wherein said one or more of the up/down-linktelescopes of a given satellite are configured to track at least tworespective ground optical telescopes of the given ground site, using anoptical beacon.
 5. The system of claim 1, wherein a single up/down-linktelescope of the given satellite is configured to track said at leasttwo respective ground optical telescopes of the given ground site usingcircular polarization or spectral diversity.
 6. The system of claim 1,wherein four inter-satellite optical telescopes are used for opticalcommunication with two nearest neighboring satellites, one of each side,and two next nearest neighboring satellites, one on each side,respectively.
 7. The system of claim 6, wherein each optical telescopeused for nearest neighboring satellite includes an aperture of about 15cm in diameter, and each optical telescope used for next nearestneighboring satellite includes an aperture of about 30 cm in diameter.8. The system of claim 1, wherein each inter-satellite telescopeincludes an aperture of about 30 cm in diameter.
 9. The system of claim1, wherein the constellation of the plurality of satellites is launchedinto a common Medium Earth Orbit (MEO) equatorial orbit plane.
 10. Thesystem of claim 1, wherein the constellation of the plurality ofsatellites includes a plurality of Medium Earth Orbit (MEO) satellitesand one or more Geostationary Earth Orbit (GEO) satellites.
 11. Thesystem of claim 10, wherein each of the plurality of MEO satellitesincludes one or more GEO-link telescopes with a field of regardsufficiency large such that at least one MEO satellite is always able tocommunicate with any of the GEO satellites.
 12. The system of claim 1,wherein the constellation of the plurality of satellites includes aplurality of Medium Earth Orbit (MEO) satellites and one or more LowEarth Orbit (LEO) satellites.
 13. The system of claim 1, wherein eachinter-satellite optical telescope includes beam steering mirrors tocompensate for jitter and orbit differences of said each inter-satelliteoptical telescope.
 14. The system of claim 1, wherein each up/down linkoptical telescope includes dual internal steering mirrors to maintaintrack on said at least two respective ground optical telescopes of thegiven ground site.
 15. The system of claim 1, wherein each up/down linkoptical telescope is configured to utilize Dense Wavelength DiversityMultiplexing (DWDM) to provide bi-directional links with at least 100Gbps bandwidth with each ground site.
 16. The system of claim 1, whereineach up/down link telescope includes an aperture of about 10 cm indiameter.
 17. The system of claim 1, wherein the plurality of up/downlink optical telescopes are gimbaled telescopes or telescopes withcoelostats on each satellite to track a ground site.
 18. The system ofclaim 1, wherein each of the plurality of satellites further includesone or more main lasers for amplifying the outgoing optical signals. 19.The system of claim 1, wherein each of the plurality of satellitesincludes multiple amplifier chains capable of reamplifying the incomingoptical signal.
 20. The system of claim 1, wherein at least one of theplurality of up/down optical telescope is available for opticalcommunication with a ground site when an optical ground connection froma preceding satellite in the constellation is nearing an end, toestablish a new connection to a following satellite in the constellationbefore breaking the optical ground connection from the precedingsatellite.
 21. A method for free space optical communication in aconstellation of a plurality of satellites, the method comprising: usinga plurality of inter-satellite optical telescopes in each of theplurality of satellites to optically communicate with multipleneighboring satellites; using a plurality of up/down link opticaltelescopes in each of the plurality of satellites to opticallycommunicate with multiple ground sites, each ground site having two ormore ground optical telescopes; tracking, by a given satellite, at leasttwo ground optical telescopes of a given ground site; determining a lineof site for each of the at least two ground optical telescopes of thegiven ground site; and sending data to a ground optical telescope withthe clearest line of sight to the given satellite, wherein each of theplurality of satellites optically processes and switches incoming andoutgoing optical signals, without converting the optical signals intoelectrical signals.
 22. The method of claim 21, further comprisingutilizing circular polarization or spectral diversity to provide dualoptical signal paths, by each inter-satellite optical telescope.
 23. Themethod of claim 21, further comprising continuously and in real timetracking at least two respective ground optical telescopes of the givenground site, by one or more of the up/down-link telescopes of the givensatellite.
 24. The method of claim 21, further comprising using fourinter-satellite optical telescopes to optically communicate with twonearest neighboring satellites, one of each side, and two next nearestneighboring satellites, one on each side, respectively.
 25. The methodof claim 21, wherein the constellation of the plurality of satellites islaunched into a common Medium Earth Orbit (MEO) equatorial orbit plane.26. The method of claim 21, wherein the constellation of the pluralityof satellites includes a plurality of Medium Earth Orbit (MEO)satellites, and one or more Geostationary Earth Orbit (GEO) satellitesor Low Earth Orbit (LEO) satellites.
 27. The method of claim 21, furthercomprising making available at least one of the plurality of up/downoptical telescope for optical communication with a ground site when anoptical ground connection from a preceding satellite in theconstellation is nearing an end, to establish a new connection to afollowing satellite in the constellation before breaking the opticalground connection from the preceding satellite.